High pressure sodium lamp (HPS) may have an elongated arc tube being enclosed within an evacuated glass cover, wherein the arc tube houses the HPS lamp's electrodes. The HPS lamp has thus a vacuum inside the glass cover (glass bulb) to isolate the arc tube from changes in the ambient temperature. The arc tube may be made of a translucent oxide and a strong discharge takes place under high temperature and pressure. The arc tube's electrodes are connected to the lamp base via conductors, provided within the glass cover.
HPS lamps are available in wattages from 35 up to 1000 watts, but the most common wattages are lying between 50 to 400 watts. One 1000 watt HPS lamp can alone produce over 140 000 lumens, with a light efficiency greater than 150 lm/W. A regular HPS lamp requires between 2500 and 4000 V starting pulse to ignite. The standard operating conditions for HPS lamps in an AC-voltage network require a supply voltage of 230 V/50 Hz. HPS lamps are in general very sensitive for deviations in the main voltage supply.
A HPS lamp is disclosed in U.S. Pat. No. 4,333,032. This HPS lamp is designed to solve the problem with sodium depletion with the arc tube, shortening the life of the lamp. The construction of U.S. Pat. No. 4,333,032 has a barium film disposed on the inner wall of the glass cover at a predetermined distance, attracting photoelectrons to the lamps lead-in conductor instead of to the arc tube.
The object of the present invention is also to achieve a HPS lamp with a long life performance. It is also an object to provide a HPS lamp which ensures that the critical lighting applications will stay lit, even after momentary power outages. Another object is also to provide a HPS lamp that ensures a lower incline of the light output and a HPS lamp involving an increased color rendering.
The object of the present invention is thus to overcome the drawbacks of known techniques.